1. Field of the Invention
The present invention relates to an optical packet processing apparatus wherein an optical coupler joins optical packets transmitted through a plurality of input side optical transmission lines and outputs the joined optical packets to an output side optical transmission line. More particularly, it relates to an optical packet processing apparatus in which even asynchronous optical packets can be fast joined as optical signals left intact, with the increase of a circuit scale suppressed.
2. Description of the Related Art
In recent years, optical communication systems which perform data communications by using optical signals have come into wide use. In such optical communication systems, packet communications are being performed likewise to those of data communications based on electric signals. The “packet communications in the data communications” are communications in which data as disassembled into cells called “packets” are transmitted and received.
A packet switch (also called “network switch”) which is employed in the optical packet communications, has a plurality of optical transmission lines connected thereto and outputs inputted optical packets to a desired optical transmission line. Accordingly, the packet switch joins the optical packets from the two or more optical transmission lines, to the single optical transmission line, and it switches the paths of the optical packets and outputs the optical packets to the desired optical transmission line. Here, in joining the optical packets, it becomes important to prevent the collision between the optical packets. Incidentally, a device which processes optical packets as optical signals left intact, without converting the optical signals into electric signals, is called “optical packet switch (optical network switch)” (refer to, for example, JP-A-06-085844).
FIGS. 7A and 7B are diagrams showing the configuration of a related-art example of an optical packet processing apparatus which joins optical packets. FIG. 7A illustrates a situation before the optical packets 1 and 2 are joined, while FIG. 7B illustrates a situation after the optical packets have been joined.
Referring to FIGS. 7A and 7B, the optical packet 1 is inputted from an input port Pi1 and is transmitted through a first input side optical transmission line 100. The optical packet 2 is inputted from an input port Pi2 and is transmitted through a second input side optical transmission line 200. An optical coupler 10 is, for example, a photocoupler, and it joins the optical packets 1 and 2 from the respective input side optical transmission lines 100 and 200, to an identical output side optical transmission line 300. The output side optical transmission line 300 is connected to an output port Po1. Incidentally, it is assumed that a time at which the optical packet 1 arrives at the optical coupler 10 from the input port Pi1 is equal to a time at which the optical packet 2 arrives at the optical coupler 10 from the input port Pi2.
The operation of such an apparatus will be described.
The respective optical packets 1 and 2 are inputted at substantially the same times from the input ports Pi1 and Pi2. Besides, as shown in FIG. 7A, the respective optical packets 1 and 2 are transmitted through the optical transmission lines 100 and 200. Further, the optical packets 1 and 2 are joined by the optical coupler 10 and are transmitted through the optical transmission line 300 until they are outputted from the output port Po1. On this occasion, as shown in FIG. 7B, it is possible that the optical packets 1 and 2 will collide to destroy data, so normal packet communications might fail to proceed.
FIG. 8 is a diagram showing the configuration of another optical packet processing apparatus in the related art. Identical reference numerals are assigned to the same constituents as in FIGS. 7A and 7B, and they shall be omitted from description. Referring to FIG. 8, a repeater 20 is disposed instead of the optical coupler 10. The repeater 20 (including a network switch or a network router) includes a plurality of optical ports 21, to which optical transmission lines 100, 200 and 300 are respectively connected. Packet processing section 22 is interconnected with the respective optical ports 21. A storage area (for example, RAM) 23 is interconnected with the packet processing section 22.
The operation of such an apparatus will be described. Respective optical packets 1 and 2 are inputted from the optical transmission lines 100 and 200 to the individual optical ports 21 at substantially the same times. The optical packets 1 and 2 of optical signals as received by the optical ports 21 are respectively converted into packets 1′ and 2′ of electric signals, which are outputted to the packet processing section 22. Besides, the packet processing section 22 once stores at least one of the packets 1′ and 2′ converted into the electric signals, in the storage area 23. The packet processing section 22 outputs the packet 1′ earlier to the optical port 21 to which the optical transmission line 300 is connected. Upon lapse of a predetermined time period, the packet processing section 22 reads out the packet 2′ from the storage area 23 and outputs it to the optical port 21. Further, the optical port 21 reconverts the respective packets 1′ and 2′ of the electric signals into the optical packets 1 and 2 of the optical signals, which are outputted to the optical transmission line 300. That is, a time adjustment is made using the storage area 23, and the collision between the optical packets 1 and 2 is avoided.
FIG. 9 is a diagram showing the configuration of another optical packet processing apparatus in the related art. Identical reference numerals and signs are assigned to the same constituents as in FIGS. 7A and 7B, and they shall be omitted from description. Referring to FIG. 9, an optical switch SW1 is disposed on an optical transmission line 100, and an optical packet transmitted through the optical transmission line 100 is inputted to this optical switch. An optical transmission line 101 is connected to one output side of the optical switch SW1, while an optical transmission line 102 is connected to the other output side of the optical switch SW1.
An optical switch SW2 is disposed on an optical transmission line 200, and an optical packet transmitted through the optical transmission line 200 is inputted to this optical switch. An optical transmission line 201 is connected to one output side of the optical switch SW2, while an optical transmission line 202 is connected to the other output side of the optical switch SW2.
An optical coupler 11 is disposed instead of the optical coupler 10, and it joins the optical packets transmitted through the optical transmission lines 101 or 102 and 201 or 202, to an identical output side optical transmission line 300. The optical transmission line 102 has a transmission line length (from the optical switch SW1 to the optical coupler 11) which is greater than that of the optical transmission line 101, and it delays the optical packet in correspondence with one optical packet. Besides, the optical transmission line 202 has a transmission line length (from the optical switch SW2 to the optical coupler 11) which is greater than that of the optical transmission line 201, and it delays the optical packet in correspondence with one optical packet.
The operation of such an apparatus will be described.
In case of delaying the optical packet which is inputted from an input port Pi1 and which is transmitted through the optical transmission line 100, the optical switch SW1 selects the path of the optical transmission line 102. On the other hand, in a case where the optical packet is not to be delayed, the optical switch SW1 selects the path of the optical transmission line 101. Likewise, in case of delaying the optical packet which is inputted from an input port Pi2 and which is transmitted through the optical transmission line 200, the optical switch SW2 selects the path of the optical transmission line 202. On the other hand, in a case where the optical packet is not to be delayed, the optical switch SW2 selects the path of the optical transmission line 201.
Besides, the optical coupler 11 joins the optical packets transmitted through the optical transmission lines 101 or 102 and 201 or 202, to the optical transmission line 300 so as to output the joined optical packets from an output port Po1. That is, the optical switches SW1 and SW2 select the paths so as to delay the optical packets from the respective input ports Pi1 and Pi2 at good timings, whereby the collision between the optical packets is avoided.
Subsequently, the operation of the apparatus shown in FIG. 9 will be concretely described with reference to FIGS. 10A to 12C. FIGS. 10A to 12C are diagrams showing the relative positional relations or relative temporal relations among optical packets 1 to 4. FIGS. 10A, 11A and 12A show states before the optical packets are inputted to the optical switches SW1 and SW2, FIGS. 10B, 11B and 12B show states before they are inputted to the optical coupler 11, and FIGS. 10C, 11C and 12C show states after they have been joined by the optical coupler 11.
First, the case of FIGS. 10A to 10C will be described. The optical packets 1 to 3 are successively inputted from the input port Pi1 in this order, and are transmitted through the optical transmission line 100. On the other hand, the optical packet 4 is inputted from the input port Pi2 and is transmitted through the optical transmission line 200. Incidentally, the optical packets 2 and 4 are synchronized and inputted (refer to FIG. 10A).
When the optical packets are joined as they are, the optical packets 2 and 4 collide. Therefore, the optical switch SW1 outputs the optical packet 1 to the optical transmission line 101 and outputs the optical packets 2 and 3 to the optical transmission line 102 in order to delay them. On the other hand, the optical switch SW2 outputs the optical packet 4 to the optical transmission line 201. Thus, as shown in FIG. 10B, the delay corresponding to one optical packet as is based on the difference of the transmission line lengths of the optical transmission lines 101 and 102 develops between the optical packets 1 and 2.
Further, the optical coupler 11 joins the optical packets 1 to 4 to the optical transmission line 300. On this occasion, an interval exists between the optical packets 1 and 2, and the optical packet 4 enters the interval, so that the optical packets 1 to 4 are joined without colliding (refer to FIG. 10C).
Subsequently, the case of FIGS. 11A to 11C will be described. Although the optical packets 1 to 4 are inputted as in FIGS. 10A to 10C, the optical packets 2 and 4 are not synchronized, and the optical packets 1 and 4 are overlappingly inputted (refer to FIG. 11A). Besides, the optical switches SW1 and SW2 switch the paths as in FIGS. 10A to 10C, and the delay corresponding to one optical packet is inserted between the optical switches 1 and 2 as shown in FIG. 11B (refer to FIG. 11B).
Further, the optical coupler 11 joins the optical packets 1 to 4 to the optical transmission line 300. On this occasion, the temporal relation between the optical packets 1 and 4 has not changed, and the optical packets 1 and 4 collide (refer to FIG. 11C).
Subsequently, the case of FIGS. 12A to 12C will be described. By the way, in FIG. 9, an optical switch, and an optical transmission line for delaying the optical packet in correspondence with one optical packet are disposed on the optical transmission line 102 anew.
The optical packets 1 to 4 which are not synchronized are inputted as in FIGS. 11A to 11C (refer to FIG. 12A) The optical switch SW1 outputs all the optical packets 1 to 3 to the optical transmission line 102, and the optical switch disposed at the succeeding stage outputs these optical packets to the optical transmission line for the further delay. On the other hand, the optical switch SW2 outputs the optical packet 4 to the optical transmission line 201. Thus, as shown in FIG. 12B, each of the optical packets 1 to 3 is delayed in correspondence with two optical packets in total.
Further, the optical coupler 11 joins the optical packets 1 to 4 to the optical transmission line 300. On this occasion, since the optical packets 1 to 3 are conspicuously delayed relative to the optical packet 4, the optical packets 1 to 4 are joined without colliding. However, although the collision has been avoided, the turns of the joined optical packets 1 and 4 are reversed (refer to FIG. 12C).
JP-A-06-085844 is referred to as a related art.
In the case where the repeater 20 for converting the optical signals into the electric signals is employed for the optical packet processing apparatus as shown in FIG. 8, the optical packets 1 and 2 can be joined at the desired timings. Since, however, the packet processing is electrically performed using the packet processing section 22 and the storage area 23, processes for converting the optical packets 1 and 2 of the optical signals into the electric signals and for restoring the electric signals into the optical signals are required.
Accordingly, problems as stated below are posed.    (1) In the repeater 20, the optical packets 1 and 2 being the optical signals are converted into the electric signals, so that a predetermined time period is expended on the conversion, and a delay occurs in the transfer of the optical packets 1 and 2.    (2) As compared with the signaling speed of optical communications, the processing speed of electric processing is much lower, so that a long delay occurs when the collision of the optical packets 1 and 2 is avoided by the electric processing.    (3) In a case where an optical packet passes through a plurality of repeaters 20, the delays in the above items (1) and (2) are involved every repeater 20, so that a long time is expended on the repeaters 20, and a long transfer delay occurs.    (4) In order to realize an electric processing speed following up the signaling speed of optical signals, equipment for electric processing needs to be enlarged in scale, and this is not practical in the points of rise in cost, increase in consumption power and enlargement in the size of an apparatus.
On the other hand, in the case where all the optical packets are joined as the optical signals left intact, as shown in FIG. 9, the synchronization among all the optical packets 1 to 4 is necessitated, or the multistage delay section (the optical switch and the delaying optical transmission line) is necessitated.    (5) In the case of employing the delay section shown in FIG. 9, the synchronization among the optical packets 1 to 4 is indispensable, and it is difficult to avoid the collision between the asynchronous optical packets 1 to 4. To begin with, in an optical communication system, it is difficult in itself to perform synchronization among a plurality of input ports.    (6) Even in the apparatus shown in FIG. 9, the collision between the asynchronous optical packets 1 to 4 can really be avoided by combining the multistage delay section. The multistage delay section, however, incurs complication in a control circuit and increase in a circuit scale and leads to such problems as rise in cost, increase in consumption power and enlargement in the size of the apparatus.